Hydrogenated nitrile rubber (“HNBR”), is generally prepared by the selective hydrogenation of a nitrile rubber (“NBR”) which is a co-polymer comprising repeating units of at least one conjugated diene, at least one α,β-unsaturated nitrile and optionally further comonomers. HNBR represents a specialty rubber which has very good heat resistance, excellent ozone and chemical resistance, as well as excellent oil resistance. Coupled with the high level of mechanical properties of the rubber, in particular the high resistance to abrasion, it is not surprising that NBR as well as HNBR have found widespread use in the automotive (seals, hoses, bearing pads) oil (stators, well head seals, valve plates), electrical (cable sheathing), mechanical engineering (wheels, rollers) and shipbuilding (pipe seals, couplings) industries, amongst others.
In general commercially available HNBR has a Mooney viscosity (ML 1+4 @100° C.) in the range of from 55 to 120 (determined using ASTM test D1646), a molecular weight Mw in the range of from 200,000 to 500,000, a polydispersity greater than 3.0 and a residual double bond (RDB) content of up to 18% (determined by IR spectroscopy).
In recent times it has been disclosed in WO-A-02/100941 and WO-A-02/100905 that a low Mooney HNBR may be prepared by using a combination of a metathesis reaction and a subsequent hydrogenation. Such low Mooney HNBR has a Mooney viscosity (ML 1+4 @100° C.) in the range of from 2 to 50, a molecular weight Mw in the range of from 30.000 to 250.000, and a polydispersity index of typically less than 3.
The principle technique of hydrogenating NBR is known to any artisan and well described in literature. In Macromolecules, 1987, 20, 2362 N. A. Mohammadi and G. L. Rempel describe the homogeneous selective catalytic hydrogenation of C═C double bounds in acrylonitrile-butadiene copolymers. Specifically examined is the hydrogenation of NBR utilizing Wilkinson's catalyst [RhCl(P(C6H5)3)3]. A spectroscopic analysis of NBR and HNBR was performed and compared in order to understand the fundamental changes to the polymer resulting from the hydrogenation process. In Rev. Macromol. Chem. Phys., 1995, C35(2), 239-285 N. T. McManus and G. L. Rempel describe catalytic hydrogenation and related reactions, covering the hydrogenation of polymers in general (NBR, SBR, polybutadiene) using several different catalyst systems, e.g. based on Rh, Ru, Pd and Tr, capable of performing polymer hydrogenation.
Presently the hydrogenation of NBR on a large commercial scale is carried out batch-wise using either homogenous or heterogeneous catalysts. Such batch-production is linked to some disadvantages. With regard to the homogeneous catalysts one disadvantage of major importance is due to the fact that the hydrogenation process of NBR is mass diffusion controlled. Therefore the ability of the homogeneous catalyst to efficiently locate and hydrogenate the double bonds of the NBR is limiting to the present batch process commercially, leading to very high manufacturing costs. Additionally the diffusion problem also applies to the hydrogen, as it is only slightly soluble in monochlorobenzene, which is the predominantely used solvent in such hydrogenation. To get enough hydrogen into the solution to perform the hydrogenation reaction requires massive pressure in the range of from 65 to 90 bar.
Additionally the most obvious characteristic of a batch hydrogenation reactor is that it produces HNBR batches which results in a specific quantity, usually dictated by the process equipment. If customers want smaller quantities than the batch size then stocks and warehousing is required. Also, the introduction of new HNBR grades into the market may be a problem, as the batch size is very likely much larger than the trial quantities required. This results in either utilization of expensive warehousing or the necessity to establish small pilot plant facilities to produce smaller quantities for trial sampling.
Most batch hydrogenation reactors are utilized for the production of a variety of different HNBR grades. The need for cleaning the reactor between different batches therefore becomes an issue. Not only does cleaning take time, but there is often associated cost of material loss and the need perhaps to dispose of cleaning solvents. Therefore, companies tend to minimize grade changes and once more there is a demand to build product inventory to satisfy customer requirements. New grades and products can be difficult to introduce with large stocks of old material in store. Furtheron, warehouses are expensive not just due to the building and operation overhead expenses but also due to the amount of working capital tied up.
The current batch-wise hydrogenation has the additional disadvantage that the reaction needs to be carried out at very high hydrogen pressures, e.g. at a pressure above 80 bar. This results in extensive and expensive safety requirements that need to be met by both the reactor and surrounding equipment.